This invention relates to electronic digital communication systems and more particularly to receivers in wireless communication systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS and WCDMA standards. This application focusses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems, including fourth generation (4G) systems that are under discussion and development.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. Since all users share the same radio frequency (RF) resource in CDMA systems, it is important that each physical channel does not use more power than necessary if system capacity is not to be wasted. This is achieved by a transmit power control (TPC) mechanism, in which, among other things, base stations send TPC commands to users in the downlink (DL) direction and the users implement the commands in the uplink (UL) direction and vice versa. The TPC commands cause the users to increase or decrease their transmitted power levels by increments, thereby maintaining target signal-to-interference ratios (SIRs) for the dedicated physical channels (DPCHs) between the base stations and the users. The DPCHs include dedicated physical data channels (DPDCHs) and dedicated physical control channels (DPCCHs) in the UL and the DL. A DPDCH carries higher-layer network signaling and possibly also speech and/or video services, and a DPCCH carries physical-layer control signaling (e.g., pilot symbols/signals, TPC commands, etc.). WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a communication system such as a WCDMA system that includes a base station (BS) 100 handling connections with, in this example, four mobile stations (MSs) 1, 2, 3, 4. In the downlink, BS 100 transmits to each mobile station at a respective power level, and the signals transmitted by BS 100 are spread using orthogonal code words. In the uplink, MS 1-MS 4 transmit to BS 100 at respective power levels. Each BS, which is called a Node B in 3GPP parlance, in the system serves a geographical area that can be divided into one or more cell(s). The BSs are coupled to corresponding radio network controllers (RNCs, not shown in FIG. 1) by dedicated telephone lines, optical fiber links, microwave links, etc. An RNC directs MS, or user equipment (UE), calls via the appropriate BSs, and the RNCs are connected to external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
WCDMA is designed to operate at low signal-to-noise ratios (SNRs) or SIRs, and therefore the WCDMA algorithms, for instance, SIR estimation algorithms and automatic frequency control (AFC) algorithms, are designed for such scenarios. It will be understood that SNR and SIR are substantially interchangeable in a communication system such as a CDMA system in which interferers (e.g., other users) are noise-like. For example, the SIR estimation algorithm, which is used in the TPC scheme to achieve sufficient quality of service (QoS), is designed to be used at low SIRs. QoS is often quantified by block error rate (BLER). It will be understood that, in WCDMA systems (and other communication systems that employ direct-sequence (DS) spread-spectrum techniques), the noise (N) includes thermal noise and interference because the spreading of the signals makes interference signals appear noise-like (i.e., spread out in frequency and with a level in the noise floor) due to the interference signals' “wrong” spreading codes.
Power control in most modern CDMA communication systems is handled by a combination of an outer loop TPC and an inner loop TPC. The SIR is used for the inner loop because it is assumed to have an almost one-to-one mapping to the BLER. The outer loop, which operates with a slower response rate than the inner loop, compensates for residual mismatch between the SIR and the BLER. TPC and SIR-to-BLER mapping are well known in the art, and are described in, for example, U.S. Patent Application Publication No. US 2005/0143112 by Jonsson, U.S. Pat. No. 6,771,978 to Kayama et al., and Louay M. A. Jalloul et al., “SIR Estimation and Closed-Loop Power Control for 3G”, Proc. 58th Vehicular Technology Conf., pp. 831-835, IEEE, Orlando, Fla. (October 2003).
In a communication system such as that depicted by FIG. 1, the BS transmits predetermined pilot symbols on the UE's DPCH. The BS also transmits pilot symbols on a common pilot channel (CPICH), and a UE typically uses the CPICH pilot symbols in estimating the impulse response of the radio channel to the BS. It will be recognized that the UE uses the CPICH pilots for channel estimation, rather than the DPCH pilots, due to the CPICH's typically higher SNR. The UE uses the DPCH pilots mainly for SIR estimation, i.e., for DL TPC.
For example, Section 5.2.3.1 of 3GPP TS 25.214, “Physical Layer Procedures (FDD) (Release 6)”, ver. 6.3.0 (September 2004) specifies that the UE shall generate TPC commands to control the network transmit power and send them in a TPC field of the UL DPCCH. Annex B.2 of TS 25.214 describes the UE's generating TPC commands for the DPCCH/DPDCH based on an estimate of the actual SIR and on a SIR reference, or target. The SIR estimate SIRest is used with the SIR target SIRref by the UE to generate TPC commands according to the following rule:    if SIRest>SIRref, generate a TPC command requesting a power decrease, and if SIRest<SIRref, then generate a TPC command requesting a power increase.
It has been observed that the response of the outer loop TPC can become slow for low BLER reference, or target, levels. Such levels are commonly called “BLER targets”. Many outer loop algorithms in today's wireless communication systems use variants of a so-called “Jump Algorithm”, which can be expressed by the following set of equations:SIRref(k+1)=SIRref(k)+SIRinc, if CRC is not OK, andSIRref(k+1)=SIRref(k)−SIRinc·BLERref/(1−BLERref), if CRC is OKin which SIRinc is a SIR increment value; k is an index that is usually equivalent to time or time slot; BLERref is a BLER target; and CRC is a cyclic redundancy check value that is included in the received block at index k and is used to determine whether the block is correctly received or not.
The BLER target is usually chosen by higher layers in the system to be low to minimize the BLER, but the Jump Algorithm has a very slow “down convergence”, especially for low BLER targets. For example, if SIRinc=1 dB and BLERref=0.01, then each downward step is fixed at only 0.01 dB. This typical behavior of the Jump Algorithm is illustrated in FIG. 2, which shows the SIR target as a “sawtooth” function of time and in which occurrences of block errors are indicated by Xs. The behavior of the typical Jump Algorithm can be seen to waste system capacity by allowing a channel to use higher SIR targets for longer times than are necessary, which is indicated in FIG. 2 by the dashed line.
There are proposals to improve such slow down convergence by using cascade controllers, where a “middle” power control loop is used to react faster to changed propagation conditions than the outer loop. The middle loop can for instance be based on bit error rate (BER), soft value quality, etc. These techniques also may have faster convergence in general. Nevertheless, there is a need for methods of faster-converging outer loop power control.